Ethylene and propylene (light olefins) are commercially important chemicals. Ethylene and propylene are useful in a variety of processes for making plastics and other chemical compounds.
One important source of light olefins is based on the pyrolysis, e.g., the steam and catalytic cracking, of selected petroleum feed materials. These procedures also produce significant quantities of other hydrocarbon products.
Converting light hydrocarbons such as methane to high value olefins such as ethylene is very economically attractive. In conventional pyrolysis processes, some of the feed methane is burned to achieve temperatures high enough to convert the methane, but yielding low carbon efficiency due to inefficient control of the reaction time.
In conventional processes, methane can be converted to acetylene using either a one- or two-step process. An example of a one-step partial oxidation process developed by BASF is described in U.S. Pat. Nos. 5,824,834 and 5,789,644. The general reactor configuration and design are described in U.S. Pat. No. 5,789,644. Acetylene can also be produced using two-stage high temperature pyrolysis, and an example two stage reactor developed by HOECHST is described in Great Britain Patent Application Publication Nos. GB 921,305 and GB 958,046.
In conventional processes, an air separation unit can be used to separate oxygen from nitrogen. The oxygen or an oxygen containing stream, along with natural gas (composed primarily of methane), are preheated and enter a partial oxidation reactor. In the BASF one stage reactor, the hydrocarbon feed and oxygen rich gas are mixed and passed through a burner block which is used to stabilize the flame that results in partial oxidation of the mixture. Secondary oxygen can be injected at the burner block to create pilot flames. The burning converts approximately one-third of the methane to acetylene, while most of the remainder is used to produce heat and lower valued products such as CO and CO2. The residence time required for the reaction process is less than 100 milliseconds. In the two stage reactor, natural gas or other fuel is mixed with an oxygen rich stream and burned in a combustion zone. The combustion products are then mixed with a feedstock of natural gas or other hydrocarbons which react to form acetylene. Again, a reaction time of less than 100 milliseconds is used. After the desired residence time, the reacting gas is quenched with water. The cooled gas contains large amounts of carbon monoxide and hydrogen as well as some carbon soot, carbon dioxide, acetylene, methane, and other gases.
Next, the gas passes through a water scrubber to remove the carbon soot. The gas then passes through a second scrubber in which the gas is sprayed with a solvent, such as N-methylpyrrolidone, which absorbs the acetylene.
The solvent is then pumped into a separation tower, and the acetylene is boiled out of the solvent and removed at the top of the tower as a gas, while the solvent is drawn out of the bottom.
The acetylene can be used to make a variety of useful products. One such product is ethylene, which can be produced by catalytically hydrogenating acetylene. A process for hydrogenating acetylene to ethylene in the presence of a Pd/Al2O3 catalyst is described in U.S. Pat. No. 5,847,250. A process for hydrogenating acetylene over a palladium-based catalyst using a liquid solvent, such as N-methylpyrrolidone, is described in U.S. Patent Application Publication Nos. 2005/0048658 and 2005/0049445.
Other known processes for converting methane to ethylene can be found in U.S. Pat. No. 7,208,647 to Synfuels International.
Controlling the reaction time is important to improve the carbon efficiency of the methane pyrolysis process. As such, technology to improve carbon efficiency is desired. Also, in addition to the main product acetylene, methane pyrolysis produces a large amount of byproducts such as carbon monoxide, hydrogen, carbon dioxide, and other gases. The economics of the process can be highly dependent on proper utilization of these byproducts.
Another, more recent source of light olefins is the oxygenate to olefins conversion process, and specifically the methanol-to-olefins (MTO) process. The MTO process is more effective in producing light olefins than conventional hydrocarbon pyrolysis systems. Instead of using a hydrocarbon source, this process is based on converting an oxygenate, such as methanol, ethanol, n-propanol, isopropanol, methyl ethyl ether, dimethyl ether, diethyl ether, di-isopropyl ether, formaldehyde, dimethyl carbonate, dimethyl ketone, acetic acid, and mixtures thereof, and preferably methanol to olefins in the presence of a molecular sieve catalyst.
There is a need for a more efficient ways to produce greater yields of light olefins, and especially propylene, from hydrocarbon feed materials.